General Background
Monoplace Hyperbaric Chambers are pressure vessels intended for human occupancy with the capacity of fully enclosing one single person for the purpose of submitting the subject to an oxygen treatment at pressure higher than 1 atmosphere absolute (ATA). Such chambers have in the past been provided with mechanical and/or electromechanical means of supplying oxygen at increasingly higher concentrations and pressure to the subject.
A hyperbaric treatment typically consists of three phases: pressurization, maintenance, and depressurization. In monoplace hyperbaric chambers, initiate the pressurization phase, a subject is placed in the chamber and the chamber door is closed. The chamber at that point is full of atmospheric air. Oxygen is supplied to the chamber at a relatively high flow rate and the chamber atmosphere is vented at a slightly lower rate, thereby causing the pressure in the chamber to increase at a pre-set rate.
Once chamber pressure reaches prescribed oxygen pressure value, the chamber pressure is maintained with a constant supply of oxygen and chamber atmosphere is vented at the same rate—this is the maintenance phase, also known as treatment plateau.
Once the prescribed, session time elapses, the chamber is depressurized and the subject is removed from the chamber.
Current state of the art chamber sessions are controlled by chamber pressure and time, and do not take into consideration the actual oxygen concentration level variation from 21% to the desired 100% within the chamber atmosphere during the session, resulting in a treatment dosage that is, at best, an approximation of the inspired oxygen percentage.
The laws of physics define that the pressure of any non-reacting gas mix results from the addition of the pressures of its components—this is known as Dalton's law. The pressure of an individual gas component in a mixture of gasses is referred to as that gas's partial pressure. The pressure of a gas mixture is the sum of the partial pressures of the individual components of the gas mixture.
This law is of the utmost importance when considering the chemical and physiological effects of gases on mammalian systems. Atmospheric air contains roughly 20.9% O2. At sea level and standard humidity and temperature, the partial pressure of the O2 component is approximately 0.209 ATA. The remaining balance of the air pressure equaling approximately 0.791 ATA consists of the sum of the partial pressures of the balance of the air's components, primarily nitrogen, but also argon, carbon dioxide, and all other trace gases.
As a functional example, the air at the peak of a Himalayan mountain has most probably the same percentage of each component, however, because the atmospheric pressure at altitude is substantially lower, so are the partial pressures of the air's components. Most mountaineers require a personal oxygen supply at these elevated altitudes because the partial pressure of oxygen at altitude may approach the 0.14 ATA critical range, which is inadequate for a human's metabolic needs. In general, air becomes hypoxic once the ambient pressure is low enough to result in an O2 partial pressure (pO2) of less than 0.18 ATA. If the pO2 decreases to 0.14 ATA, humans quickly become hypoxic and may die after just a short period of time. On the other hand, air becomes hyperoxic when the ambient pressure creates a pO2 exceeding 0.23 ATM.
Reverse examples can be given and illustrated with SCUBA diving, wherein humans are subjected to elevated pressures. For example, when breathing pure oxygen, a subject is limited to the maximum depth of 6 meters (18 Ft) because the oxygen partial pressure at this depth is 1.6 ATA, corresponding to a pO2 value considered to be the maximum for underwater activities, for the elevated pO2 can potentially cause central nervous system toxicity resulting in convulsions, which would likely be deadly underwater. The same principle dictates that approximately 65 meters (˜213 ft.) is the maximum depth for subjects breathing regular air (˜21% O2), for at this depth, the pO2 is the same as breathing pure O2 at 6 meters.
The above examples serve the purpose of illustrating that the physiological effects of oxygen depend on the respective pO2 and not on the percentage of O2 in the breathing gas mix alone.
Hyperbaric chambers (or simply “chambers”) are used for medical purposes to reduce and/or eliminate numerous diseases and ailments. Without exception, all currently manufactured monoplace hyperbaric chambers using full body oxygen pressurization have a manual or manual/automatic control system. In all known cases, the hyperbaric session is controlled, by two parameters: session time and chamber absolute pressure which are prescribed by an MD. In these cases, session time (also referred to as treatment time) is defined as the time counted from the start of chamber pressurization to start of chamber depressurization. Oxygen pressure is often confused with absolute pressure of the chamber.
Referring to FIG. 1, this graph exemplifies what a physician may prescribe in the prior art for a subject: 60 minutes at 2 ATA O2. This prescription is represented by the shaded area of the graph in FIG. 1.
FIG. 2 illustrates the theoretical effective portion of the prior art hyperbaric treatment of FIG. 1. Once the chamber door is closed and the pressurization cycle initiated, the treatment session clock starts. The operator starts the pressurization cycle at the same time that an exhaust valve is opened to allow for the flushing of the initial volume of air in the chamber.
FIG. 3 illustrates a chamber session, as practiced in the prior art, but additionally highlighting the actual value in the session time frame. As illustrated, in excess of minutes are required to reach approximately 96% oxygen concentration, which is close to ⅓ of the total session time. At 2 ATA and 95% concentration, the pO2 is merely 1.9 ATA and not the prescribed 2 ATA of pO2.
Trend analysis indicates that several hours would be necessary to reach the 2 ATA of oxygen pressure prescribed by the physician, which is well past the end of the prescribed session, assuming a prescription for 1 hour at 2.0 ATA O2.
Additionally, the time frame before the chamber atmosphere reaches the level of 95%, oxygen concentration is fully counted as session time even though during the first 20 minutes of the session the oxygen partial pressure is clearly below the prescribed value.
Treatment efficacy also depends on chamber size, for a smaller chamber will arrive at acceptable levels of oxygen concentration, more quickly than a larger chamber, same flow rate provided, yet this is a variable not accounted for. When the chamber is first used it contains an amount of ambient air equal to the chamber internal volume, but when the subject is placed in the chamber, the air volume displaced by the subject is removed from the chamber. Therefore, the chamber possesses an air volume equal to its internal volume minus the volume displaced by the subject and any introduced equipment. This volumetric variation is ignored in traditional chamber treatments.
Oxygen flow rate is also currently used as means to control the chamber inner temperature and humidity. Given that gasified liquid oxygen is intrinsically cold, a chamber operator may opt to use a lower flow rate in colder days than in warmer days. This affects the rate at which the original air volume is flushed out and introduces yet another unknown variable in knowing the actual oxygen percentage in the inspired gas.
Overall, as illustrated by FIG. 3, at best the subject only receives approximately ⅔ of the treatment prescribed by the physician and the prescribed pO2 is unlikely to ever truly be achieved.
There is a need for a hyperbaric chamber control system that enables a subject to receive an accurate hyperbaric treatment, as prescribed by a physician.
Pressure Swing Adsorption Background
Current hyperbaric chambers often lack portability and site compliance standards are difficult to meet due to the presence of liquid oxygen. The storage of liquid oxygen represents a very high risk of explosion and fire, and the premises must be explosion proof in most jurisdictions. Since liquid oxygen is also not readily available in many areas where hyperbaric treatments are required, such as seafaring vessels, remote locations, and conflict zones, one embodiment of the present invention utilizes a pressure swing adsorption (PSA) device and method to enrich air with oxygen for use in the present embodiments of hyperbaric control.
Pressure swing adsorption (PSA) processes rely on the fact that gases under pressure tend to be attracted to solid surfaces, or adsorbed, when under pressure. The PSA oxygen enrichment process relies on a material called zeolite to remove nitrogen from ambient air. Zeolite refers to the family of aluminosilicate minerals having microporous structures capable of loosely binding with a variety of cations. Zeolite is used in oxygen enrichment, as it adsorbs nitrogen when subjected to a high pressure, yet releases the absorbed nitrogen when the pressure drops.
The PSA process works by feeding ambient air into a pressurized chamber containing zeolite. The higher the pressure within the zeolite chamber, the more gas is adsorbed. When the pressure is reduced, the gas is released. PSA processes can be used to separate gases in a mixture because different gases tend to be attracted to different solid surfaces more less strongly. If a gas mixture such as air (21% Cu, 78% N2) is passed, under pressure, through a tower containing an adsorbent bed that attracts N2 more strongly than it does O2, a large portion of the N2 will stay in the bed, and the gas leaving the tower will contain approximately 94% O2. When a bed in the first tower reaches the end of its capacity to adsorb N2, the bed can be regenerated by reducing the pressure, thereby releasing the adsorbed N2. It is then ready for another cycle of producing oxygen enriched air (OEA).
To generate purified O2 from air, N2 from the air is absorbed by zeolite while the O2 is further passed through to a storage tank. When the zeolite is saturated (so it can no longer effectively absorb N2), the chamber is depressurized. When the chamber is in the depressurized state, the N is released by the zeolite and vacates the system. The chamber is than re-pressurized and the cycle is repeated to generate purified O2.